Explanation:
standard definition :
tan(x) = sin(x)/cos(x)
sine is the vertical (up/down) leg, cosine is the horizontal (left/right) leg.
and we need to multiply every standard function by the actual radius of the circle around. this radius is the Hypotenuse of the right-angled triangle.
but we need to position abd orient the triangle correctly, so that the angle in question (here B) is the bottom left corner and therefore the center of the imaginary circle around the triangle.
so, when we turn the triangle 90° counterclockwise, and we look from the point of view of B, we see
sin(B) × 95 = 40
cos(B) × 95 = 75
tan(B) = sin(B)×95 / (cos(B)×95) = sin(B)/cos(B) =
= 40/75